The leaves of various plants including macroalgae, grasses, and sedges are known to have symptomless infections. The fungi involved are commonly referred to as endophytes. (Carroll, 1988; Clay, 1988). An endophyte is “an organism inhabiting plant organs that at some time in its life, can colonize internal plant tissue without causing apparent harm to the host” (Petrini 1991). Fungal endophytes are believed to be host specific such that they infect one or a small subset of plant species.
It is well understood that toxic metabolites produced by grass endophytes greatly reduce populations of herbivorous insects attacking the plant. This has a large affect on plant fitness (Clay, 1988; Clay & Holah, 1999). Grass seed of cultivars that contain fungal leaf endophytes has, for 10 years, been the dominant technology used for lawns and golf courses in parts of the US and Canada. These fungi produce very potent toxins inside the grass leaves that kill insects. This vastly reduces the amount of hard chemical pesticide used on the resulting lawns. Such lawns have increased drought tolerance and have increased tolerance to fungal diseases.
Conifer needles are also infected by systemic fungal endophytes that may fulfill several ecological roles (Carroll 1988; Ganley et al. 2004).
Carroll & Carroll (1978) first proposed that fungal endophytes recovered from coniferous needles might be mutualistic symbionts. They suggested decreased palatability for grazing insects and antagonism towards needle pathogens as possible benefits for the host trees. In subsequent work, this group studied the association between Douglas-fir (Pseudotsuga menziesii) and the needle endophyte Rhabdocline parkeri (Sherwood-Pike et al., 1986; Todd, 1988). An extract of R. parkeri was cytotoxic to HeLa cells and resulted in reduced growth rates and mortalities when incorporated into synthetic diets of Choristoneura fumiferana (spruce budworm) at 10 μg g−1 (Miller, 1986).
Conifers, like other plant species, are vulnerable to pest damage. For example, the eastern spruce budworm is an economically-damaging insect pest. The last time there was an epidemic in Eastern Canada, large scale spraying of a hard chemical pesticide was undertaken. Where this was not done, the forests were devastated. For the year 1977 alone, the cost of the spraying program in New Brunswick was approximately $47 million in constant dollars. Over the intervening two decades, during a low period of the budworm cycle, the hard chemical pesticides used in the 1970's were de-registered in favour of biopesticides. Regardless, it is less likely now that the social consensus would exist for the widespread use of chemical insecticides when the spruce budworm population returns to epidemic proportions. New methods of controlling spruce budworm and other insect pests are needed.
Royama (1984) published a comprehensive analysis of the population dynamics of the spruce budworm focusing on the period 1945 to 1983 in New Brunswick (NB), Canada. One feature of this analysis is that he proposed a “fifth agent” referring to an unknown factor that was required to build models that best fit observed population changes. The central characteristic of this fifth agent was that it in some way changed the response of the insect populations to known factors such as weather, predation and disease.
From 1984-1994 isolations were made of endophytes present in needles in various species of conifers across NB. As found by workers worldwide, the needles of all mature conifers examined were colonized by several species of endophytes (Johnson & Whitney, 1989; Wilson, 1994). From collections from across NB comprising 3500 strains, a low percentage from Abies balsamea (balsam fir), Picea rubens (red spruce), Picea glauca (white spruce) and Picea mariana (black spruce) were found to produce anti-insectan toxins (Calhoun et al., 1992; Clark et al., 1989 Findlay, 1996; Findlay et al., 1994; Findlay et al., 1995 One of the toxins, rugulosin, was obtained from cultures derived from red spruce needles (Calhoun et al., 1992).
In nature, tree seedlings may acquire needle endophytes from the trees surrounding the growing tree. However, most of these strains are apparently not able to produce anti-insectan compounds. Commercially produced seedlings leaving production facilities are not colonized by needle endophytes (Miller et al., 2002). Conifers inoculated with endophytes to increase pest tolerance would be highly desirable considering the hundreds of millions of seedlings produced in North America annually.
There are difficulties in colonizing conifers with endophytes. Economically viable large-scale inoculation of conifers with desirable strains of endophytes requires a method with increased colonization efficiency and ease of inoculation.